Starches and modified starches have been the focus of considerable research interest in attempts to use these as fillers in order to decrease polymer costs and to use polymers that are biodegradable. As several recent examples, U.S. Pat. No. 5,384,187, issued Jan. 24, 1995, inventors Uemura et al., U.S. Pat. No. 5,391,423, issued Feb. 21, 1995, inventors Wnuk et al., and U.S. Pat. No. 5,412,005, issued May 2, 1995, inventors Bastioli et al., all represent domestic and foreign based attempts to achieve biodegradable polymer compositions in which natural polymers such as starches have been added to synthetic polymers. Unfortunately, while the inclusion of starches can reduce costs, the mechanical properties of the synthetic polymer can be so adversely affected by inclusion of starch as to vitiate the cost advantages.
Among examples of physical strength loss when starch is blended with various synthetic polymers are those described in several recent articles. Thus, for example, Koenig and Huang, PMSE, 67, pp. 290-291 (1992) used three different types of synthetic polymers in combination with starch and starch derivatives and reported their properties. When polycaprolactone ("PCL") was filled with 25 wt. % starch or a starch derivative the yield strength and the tensile strength were reduced by half.
Ramsay et al., Applied and Environmental Microbiology, 59, pp. 1242-1246 (1993) studied poly-.beta.-hydroxyalkanoates with starch. The biodegradable polymer [P(HB-co-HV)] was said to hold biodegradable promise, but the polymer is significantly higher in price than a commodity plastic such as polyethylene or polystyrene. The inclusion of 25 wt. % granular starch was reported to result in a composition with a tensile strength of about 60% the original (16 MPa by contrast to 27 MPa). The authors acknowledged that the use of unmodified granular starch as a particulate filler in the [P(HB-co-HV)] polymer reduced the tensile strength and did not offer any appreciable reinforcement in the mechanical rigidity, presumably due to poor adhesion of the polymer granule interface. The authors concluded it would be necessary to develop formulations with improved adhesion.
Accordingly, attempts continue to find synthetic polymer based compositions that can be formed into articles, such as for example disposable plastic utensils or stretchable thin films for food packaging purposes, that are reasonably competitive in price with commodity plastics such as polyethylene or polystyrene, but which are more environmentally friendly.